A conventional belt-type continuously variable transmission is configured to change a speed change ratio and a torque capacity by varying an oil pressure or an oil amount applied to a hydraulic actuator. In case the belt-type continuously variable transmission thus structured is used in a vehicle, oil reserved in an oil pan is pumped up by an oil pump driven by a prime mover of the vehicle, and fed to the hydraulic actuator while being pressurized. In this situation, if the oil leaks from the belt-type continuously variable transmission, the oil pump has to be driven unnecessarily to feed the oil to the hydraulic actuator thereby compensating the leaking oil. In this case, therefore, a fuel economy of the vehicle is deteriorated by thus driving the oil pump more than necessary.
For example, international publication WO 2010/021218 discloses a hydraulic control device capable of reducing oil leakage from a control valve for feeding and discharging oil to/from a belt-type continuously variable transmission. Specifically, according to the teachings of WO 2010/021218, each pulley of the belt-type continuously variable transmission is connected with electromagnetic poppet valves capable of reducing oil leakage, and those valves are opened in case of maintaining an oil pressure or an oil amount applied to the pulleys.
As described, the oil reserved in the oil pan is supplied to the hydraulic actuator. However, the oil reserved in the oil pan is mixed with the oil returned to the oil pan after lubricating another frictional portion. Therefore, abrasion powder or the like is contained in the oil supplied from the oil pan to the hydraulic actuator. In order to avoid the above-explained disadvantage, Japanese Patent Laid-Open No. 8-135797 discloses a seal ring configured to discharge foreign particle while leaking a given amount of oil. Specifically, according to the teachings of Japanese Patent Laid-Open No. 8-135797, a sectional area of a flow path will not be changed even if the seal ring is thermally expanded due to a rise in the oil temperature. Therefore, the seal ring taught by Japanese Patent Laid-Open No. 8-135797 is capable of discharging the oil at a constant rate.
Thus, frequency in use of the oil pump used in the belt-type continuously variable transmission can be reduced by preventing an occurrence of the oil leakage from the hydraulic actuator as taught by WO 2010/021218. However, if the oil is sealed in the hydraulic actuator to stabilize the speed change ratio or the torque capacity, the temperature of the oil in the hydraulic actuator has to be raised and this may cause deterioration of the oil.